From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by ozlabs.org (Postfix) with ESMTPS id C5328DDFF0 for ; Wed, 28 May 2008 08:20:01 +1000 (EST) Subject: Re: MMIO and gcc re-ordering issue From: Benjamin Herrenschmidt To: Linus Torvalds In-Reply-To: References: <1211852026.3286.36.camel@pasglop> <20080526.184047.88207142.davem@davemloft.net> <1211854540.3286.42.camel@pasglop> <20080526.192812.184590464.davem@davemloft.net> <1211859542.3286.46.camel@pasglop> <1211922621.3286.80.camel@pasglop> <1211924335.3286.89.camel@pasglop> Content-Type: text/plain Date: Wed, 28 May 2008 08:19:39 +1000 Message-Id: <1211926779.3286.104.camel@pasglop> Mime-Version: 1.0 Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, tpiepho@freescale.com, linuxppc-dev@ozlabs.org, scottwood@freescale.com, David Miller , alan@lxorguk.ukuu.org.uk Reply-To: benh@kernel.crashing.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Tue, 2008-05-27 at 14:55 -0700, Linus Torvalds wrote: > > On Wed, 28 May 2008, Benjamin Herrenschmidt wrote: > > > > A problem with __raw_ though is that they -also- don't do byteswap, > > Well, that's why there is __readl() and __raw_readl(), no? As I replied to somebody else, __readl() is news to me :-) we dont' have those on powerpc. > Neither does ordering, and __raw_readl() doesn't do byte-swap. But I can add them :-) > Of course, I'm not going to guarantee every architecture even has all > those versions, nor am I going to guarantee they all work as advertised :) > > For x86, they have historially all been 100% identical. With the inline > asm patch I posted, the "__" version (whether "raw" or not) lack the > "memory" barrier, so they allow a *little* bit more re-ordering. > > (They won't be re-ordered wrt spinlocks etc, unless gcc starts reordering > volatile asm's against each other, which would be a bug). > > In practice, I doubt it matters. Whatever small compiler re-ordering it > might affect won't have any real performance impack one way or the other, > I think. I prefer explicit endian. Always. Thus I prefer introducing _be variants (we already have those on powerpc and iomap has it's own _be versions too) so we should probably generalize _be. Ben.